Compressor systems generally include a compressor for compressing gases such as air for example, a tank for receiving and storing the compressed gas, and a motor for driving the compressor. In use, compressed gas is released from the tank for such purposes as spraying paint, inflating automobile tires, or powering pneumatic tools. Normally, the compressor automatically starts and stops according to the demand of the system. When the pressure in the tank drops to a pre-selected lower level, the compressor starts in order to refill the tank, and when the tank pressure reaches a pre-selected upper level, the compressor stops.
Operation of the compressor to produce compressed gas is usually controlled by one of two methods. First, in a start/stop control mode, the motor driving the compressor is automatically enabled and disabled in response, respectively, to pre-selected lower and pre-selected upper pressure levels in the tank. Second, in a load/unload control mode, the motor is continuously run but the compressor is loaded and unloaded automatically in response, respectively, to pre-selected lower and upper pressure levels in the tank. As is known, an unloader opens a valve and/or closes an intake port to prevent the compressor from compressing gases. Thus, when unloaded, the compressor continues to run with only friction losses but no pressure is produced, such that compressor and motor wear are minimal.
Under light load conditions, i.e. light and/or infrequent demand for compressed gas, the start/stop control mode is preferable. Light load conditions are characterized by short run periods for the compressor followed by a relatively long period in which the tank does not require additional compressed gas. By stopping the motor instead of unloading the compressor, motor and compressor wear are reduced and no power is consumed during the period that the compressor is not required to run.
The load/unload control mode, in contrast, is preferable under heavy load conditions, i.e. heavy and/or frequent demand for compressed gas. Under heavy load conditions, the compressor is required to start and stop frequently and to run for extended periods. Since most motors are very inefficient during start-up, attempts to control the compressor by starting and stopping the motor result in an increased energy consumption due to the frequent starts and may possibly lead to motor damage. In contrast, if an unloader is used to control the compressor, the unloaded compressor requires little power input during the relatively short unloaded periods, and the continuous operation of the motor during the unloaded period generally requires less energy than a restart of the motor. Thus, the load/unload control mode is more efficient and economical under heavy load conditions.
Some compressors, however, operate under both heavy and light load conditions. In such cases, the compressor is usually provided with both a start/stop control mode and an unloader control mode. Selection between the control modes can be provided manually or automatically.
U.S. Pat. No. 4,863,355 to Odagiri et al. and U.S. Pat. No. 4,201,517 to Ferguson, for example, both generally disclose a control that automatically selects between a start/stop and a load/unloaded mode based upon the rate of change of tank pressure.
U.S. Pat. No. 1,521,034 to Maxson discloses a compressor control that provides a modified load/unload control mode wherein the control stops the motor after a fixed period if the rate of change of pressure is low.
U.S. Pat. No. 4,149,827 to Hofmann, Jr. discloses a method providing automatic selection of the mode of operation. Depending on the rate of change of tank pressure, the compressor operates in a start/stop, loaded/unloaded, or a "regulated" mode. In the regulated mode, the compressor intake is varied between open and closed.
U.S. Pat. No. 4,453,893 to Hutmaker discloses a drainage control for a compressor system. The control automatically purges a compressor tank of the system each time the compressor is shut off.
What is desired, however, is a controller for a compressor system that provides a start/stop control mode that allows a compressor of the system to run unloaded for a pre-selected selected period after a motor of the system is started. In addition, it is desired to have a controller that provides a load/unload control mode wherein a motor of the system is automatically stopped after a manually selected period after a compressor of the system is unloaded. It is also desired to have a controller providing these features plus an emergency stop feature wherein a motor of the system is stopped upon the controller receiving an indication that a compressor is operating improperly, or a recording of cumulative operating time feature, or an automatic tank drain feature based upon cumulative operating time.